If I'm in a relationship, I wonder about this also. Usually, I look for a few things.
1) Conversations - When you guys talk, is it always you trying to spark something to talk about? If it's always you, it could be a sign that she is bored, but it could also be a sign that she's concerned about something. It's completely up to you to figure out which one it is.
2) Assertiveness - Are you always the one asking to meet up? Does she ask to meet up with you? Are you always the first one to take action into something? I do think that in a healthy relationship, both you and your partner have an equal level of assertiveness. This, of course, is just my opinion, though. Certain people would argue that guys should always be the one making the moves and the girl should just be taking the backseat, but I disagree with this.
3) When making love - Does it feel like you're the only one enjoying? No, seriously. I do think that if a girl is bored, she's usually apathetic about the entire situation and shows no pleasure.
Anyway, those are some of the few things that you can consider. There's probably a lot more things you can think of, but hopefully I got you off on the right track.